Aircraft jet engines of current design are generally started using spark discharge igniters. These igniters may employ a high voltage (several tens of kilovolts) which is discharged across a gap formed between a pair of electrodes in a manner similar to ordinary automobile spark plugs, or may employ a low voltage (about 2 to 5 kilovolts) but high energy (10 to 12 Joules) system.
The low voltage type of igniter employs a sealed barrier gap switch to hold-off the discharge until a relatively large energy build-up has occurred. The barrier switch then fires at a predetermined voltage allowing the discharge across the air gap to proceed. The plugs generally consist of inner and outer concentric electrodes spaced by a ceramic high temperature insulator. The insulator is coated with a semiconductor material which facilitates ionization in the discharge gap, thus permitting the spark discharge to initiate at the relatively low two kilovolt voltage. The discharge starts along the surface and builds to a typical ignition spark.
One class of the high voltage type of spark discharge igniters which are in common use are cavity type plasma jet igniters. These are spark plugs that have a small cavity integrated into the region in which the electrodes are located. When fired, the arc discharge ejects or puffs-out a heated volume of gas and a discharge plasma filament.
The cavity type plasma jet spark plug type has a single orifice through which the air that is heated by the arc discharge in the cavity is forced out by thermal expansion, thereby creating a short duration jet. The plasma, which is the ion-electron streak of the spark discharge, is directed out of the cavity region toward the fuel-air mixture and ignites the mixture. Variations of this type of plug in combination with surface discharge spark initiation features have become widely used as combustor igniters in turbojet engines and have seen some application in internal combustion piston engines. But, it must be recognized that these igniters do not and cannot sustain combustion, they only ignite the flowing fuel-air mixture for the duration of each, plasma pulse. As a consequence, other means must be provided to maintain a continuous flame within the engine during operation (generally called "stabilization" of the flame).
A key advantage of cavity type plasma jet spark devices over ordinary spark plugs is that they launch the short-duration, spark-type, plasma filament away from the combustor walls, which could, otherwise cool or quench the ignition kernel. Further, by launching it into the combustor, it more readily reaches a region containing a combustible mixture. A combustible mixture in a turbojet combustor region near the walls can be in a less than optimum condition for ignition due to boundary layer mixing limitations.
On the other hand, cavity type plasma jet spark devices have significant disadvantages in aircraft jet engine applications. In order to deliver the appreciable level of energy necessary to induce ignition, they must operate intermittently. Normally, only about 1 to 2 joules of the 12 to 14 joules developed in a high energy aircraft system reaches the combustion kernel region. This limits state-of-the-art systems to less than 100 to 300 discharges per minute if minimally adequate energy is to be transferred. This is a severe disadvantage during a landing approach or under bad weather flight conditions, when assurance of engine ignition is critical. It is also a severe disadvantage when attempting to relight a flamed out engine at high altitudes.
Also, plug discharge technology is extremely sensitive to excess quantities of fuel (rich mixtures) which can be developed in emergency or inclement weather relight situations. Under these conditions, the discharge can be very substantially reduced and its plasma jet ejection capability almost eliminated.
There have been a number of plasma plume devices built in the past, mostly for use as torches. These devices have not generally been thought suitable for use as engine igniters because they have not been self-starting, requiring some sort of triggering spark to initiate the plasma.